164 On the Osteology of the Hyopotamidae. [Feb. 6, 



following the adaptive mode of reduction, means that the reduction of 

 the manus and pes was carried so far that it could not proceed further ; 

 this point was attained already in the Lower Miocene. When once the 

 metapodium was reduced to one bone, and this one had taken the whole 

 distal surface of the carpus and tarsus, any further reduction or improve- 

 ment was quite impossible. Besides, the completely developed faculty of 

 rumination gave these forms an enormous advantage over the other, non- 

 ruminant, Paridigitata occurring in the same strata. They could live on 

 such matters as twigs, bark of trees, mosses, lichens, on which no other 

 TIngulata can subsist ; such food is found everywhere, requires no c unnin g 

 and very little struggle to get it. All essential modifications were attained 

 very early, and the chief of these are the confluence of the two middle digits 

 in a complete cannonbone and rumination. Then began the luxury of all 

 sorts of appendages — excrescences on the frontal bones covered with skin, 

 uncovered by skin in the form of prickly simple horns (Pudu), or double 

 (Dicroceras of Sansan, Muntjac), then branched and palmated. In other 

 groups these bony cores were covered with horny sheaths, which at first 

 differed but little from agglutinated hairs (Antilocapra americana), then 

 became more compact, as in the smooth and hard horny sheath of the 

 hollow-homed Euminantia. These secondary characters were all acquired, 

 thanks to abundant time, after the essential characters of the type had 

 been assumed ; if man had come on earth a little later than he did, he 

 certainly would have found nearly parallel cases in the group of Suina, 

 monodactyle (with cannonbone) hogs with different appendages. As 

 it is, he stopped the course of events ; all further improvement is out 

 of the question, or only possible in such groups as the Eodentia, who 

 prey on man's food, being at the same time independent of him. 



It may be asked, How stands the matter in thelmparidigitateUngulata ? 

 And though I cannot enter fully into the case, I may state that the same 

 course of events is observable in them ; only there could be no in- 

 adaptive reduction, as the body could not, under any circumstances, be 

 held in equilibrium upon one single third digit, if this one had not 

 taken the whole distal surface of the carpus and tarsus. But the 

 task in this group was much more difficult ; to get one middle digit to 

 perform the work shared in the ancestors by five, and in the immediate 

 progenitor by three, required time. To accomplish this, two geological 

 periods were needed ; but still, by the incessant tendency to reduction, 

 the work was done, and the monodactyle horse spread over the surface of 

 the globe, superseding all other Imparidigitata, which are evidently 

 rapidly dying out. The only two genera which remain still, the Rhino- 

 ceros and the Tapir, cannot last long. But this spreading and multipli- 

 cation of the Equidce was also accompanied by a total change of diet : 

 from an omnivorous animal it became grass-eater; and indeed, by its teeth 

 and many other characters, the horse is very analogous to the Kuminantia, 



